1.Field of Invention
The present invention relates to a power converter. More particularly, the present invention relates to a transformer structure of the power converter capable of suppressing a common mode current.
2.Description of Related Art
In the prior art, a power converter including a transformer often causes a common mode noise. It is well known that, the transformer includes a primary winding, a magnet core and a secondary winding. In general, parasitic capacitances exist between the primary winding and the secondary winding, between the primary winding and the magnet core, and between the secondary winding and the magnet core. During the operation of the power converter, when a power component is repeatedly turned on or off at a high frequency, a voltage jump occurs in the primary winding and/or the secondary winding of the transformer, the voltage jump causes displacement currents through the aforementioned parasitic capacitances. When the displacement currents flow into the ground, the common mode noise is formed in the power converter.
When a voltage jump occurs in the winding of the transformer, according to the voltage jump condition, the winding terminals of the primary winding and/or the secondary winding are respectively referred to as “a static terminal” and “a jump terminal”, and respectively include a primary static terminal, a primary jump terminal and/or a secondary static terminal and a secondary jump terminal. The static terminal is a terminal where a voltage relative to earth ground does not jump or the jump frequency of the voltage relative to earth ground is smaller than the switching frequency. The jump terminal is a terminal where the jump frequency of the voltage relative to earth ground is close to or larger than the switching frequency.
In order to suppress the common mode noise formed in the power converter having the transformer, a conventional solution uses a copper foil layer to shield the common mode current between the primary winding and the secondary winding of the transformer. It is assumed that the number of turns of the primary winding is larger than that of the secondary winding, and when a voltage jump occurs, the voltage jump magnitude of the primary winding is larger than that of the secondary winding. At this time, the common mode noise caused by the voltage jump of the primary winding is dominant. By winding a copper foil shielding layer between the primary winding and the secondary winding, and electrically connecting the copper foil shielding layer to the static terminal of the primary winding (i.e., the primary static terminal), no voltage jump occurs in the copper foil shielding layer, and the electric field generated by the voltage jump of the primary winding is shielded by the shielding layer, thereby reducing the common mode current. However, the copper foil shielding layer can only be used to reduce the common mode current generated by the primary voltage jump or the common mode current generated by the secondary voltage jump, but the common mode current from the primary side to the secondary side, and the common mode current from the secondary side to the primary side cannot be suppressed or balanced essentially. Moreover, the shielding method using the copper foil is hard to be implemented by automatic processing, and the manufacture and winding of the shielding layer are both completed manually, which has disadvantages of high production cost and low efficiency.
In view of this, many in the industry are endeavoring to find ways in which to design a transformer capable of suppressing the common mode current, so as to eliminate one of the disadvantages of using the copper foil shielding layer, to enable the automatic production and reduce the production cost while further reducing the common mode noise.